During the concentration operation often used in the preparation of aqueous fluoropolymer dispersions, e.g., aqueous polytetrafluoroethylene (PTFE) dispersion, nonionic surfactants are added in order to improve dispersion stability. U.S. Pat. No. 3,037,953 discloses the preparation of concentrated PTFE aqueous dispersions using nonionic surfactants such as polyoxyethylene octyl phenyl ether and ethylene nonyl phenyl ether for use in preparing concentrated PTFE aqueous dispersions. These PTFE aqueous dispersions containing blended polyoxyethylene alkyl phenyl ethers provide dispersion stability, but disadvantages have been reported. For example, such surfactants can decompose to form harmful compounds that may have adverse health or environmental impact.
For this reason, PTFE aqueous dispersions are now being developed that contain, instead of polyoxyethylene alkyl phenyl ethers, blended polyoxyethylene alkyl ethers that do not contain benzene rings and thus do not generate harmful aromatic compounds. For example, Japanese Kokai Patent Application No. Hei 8[1996]-269285 (U.S. Pat. No. 6,153,688) discloses PTFE aqueous dispersions with cloud points of 45-85° C. for use with fibrous substrates and containing polyoxyethylene alkyl ethers that do not release harmful substances such as benzene upon degradation. This publication also mentions that these PTFE aqueous dispersions may be used as a binder in batteries. In addition, PTFE aqueous dispersions containing branched polyoxyethylene alkyl ethers with saturated alkyl groups have been disclosed in Japanese Kokai Patent Application No. Hei 11[1999]-152385 for use in the production of high-strength PTFE films with improved thickness uniformity. Japanese Kokai Patent Application No. 2002-179870 discloses that the use of polyoxyalkylene-based nonionic surfactants having alkyl groups with 16 or greater carbon atoms provide superior environmental properties and dispersion stability.
For electrode applications, e.g., battery or fuel cell electrodes, nonionic surfactants in a PTFE aqueous dispersion employed as a binder or hydrophobic agent as should possess certain properties to achieve good results. During electrode manufacture, the dispersions are combined with electrode materials and these mixtures are dried by heating. Surfactant is removed during drying step. However, since the PTFE and/or electrode materials in the electrode may be adversely affected by high temperatures, the surfactant should be sufficiently volatile that surfactant is removed without resort to excessive heating. Also, the surfactant should not be prone to degradation upon heating which can leads to deposits on the electrode. Deposits on the electrode have the potential to directly interfere with electrode function or to react with cell chemicals to form products which adversely affect the operation in the cell. It is also important for the dispersions to contain at most low levels of metal impurities that may lead to undesirable effects in electrode applications. Resistance to sedimentation and good viscosity stability are also desirable.
Polyoxyalkylene alkyl ethers, specifically polyoxyethylene alkyl ethers commonly used as nonionic surfactants, have a wide alkylene oxide (e.g., ethylene oxide) molar addition distribution relative to polyoxyalkylene alkyl phenyl ethers such as polyoxyethylene octyl phenyl ether or polyoxyethylene nonyl phenyl ether. Because their molecular weight distributions of such polyoxyalkylene alkyl ethers are extremely broad, they typically contain high molecular weight fractions of low volatility. Such fractions require high drying temperatures and are prone to thermal degradation. On the other hand, a low molecular weight fraction can also be present which, in significant amount, compromises sedimentation stability and viscosity stability. The polyoxyalkylene alkyl ethers typically used also have a high content of alkali metal and other impurities because they are generally produced using catalysts containing alkali metals such as sodium hydroxide.